This invention relates generally to a technique for converting small round logs into lumber products, and more particularly to a technique in which logs of small diameter are cut into wood pieces that are stacked to form a block assembly in which the pieces are separated from each other to define a matrix of intersticial space, the pieces in the block assembly being interlaminated by injecting the matrix with an elastomeric plastic material to form an integrated stock block that is dividable into panels each constituted by wood tiles joined together by flexible hinges, whereby the resultant panel is contourable.
A technique in accordance with the invention, though applicable to various species of wood, is of particular value in connection with balsa wood derived from a tropical American tree (Ochroma pyramidale). Balsa wood has outstanding properties unique in the lumber field; for on the average, it weighs less than 9 pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength superior to any composite or synthetic material of equal or higher density. While a technique in accordance with the invention will be described herein only in regard to balsa wood, it is to be understood that it is also applicable to other wood species.
The cost of balsa wood products has heretofore been keyed to the low yield obtainable when employing conventional techniques to convert balsa logs into usable products. The traditional conversion technique results in a low yield in that the amount of balsa convertible into usable lumber is usually less than half the total volume of wood in the log, the balance being wasted.
The economics of converting balsa logs into commercially-available lumber products must take into account a number of factors, such as growth time, kiln drying costs and the relationship of yield to tree diameter. The traditional conversion technique produces balsa lumber products from logs having a diameter of 12 inches or greater and inevitably results in products which are expensive. It not only requires about eight years before the trees can be harvested to produce logs of this size, but kiln costs are high and the yield is low in that a large percentage of the wood is wasted in the conversion process.
A marked improvement in the economics of converting balsa logs into usable products is gained by the technique disclosed in my prior U.S. Pat. No. 4,122,878, based on Ser. No. 860,617, filed Dec. 14, 1977, entitled "Technique for Converting Balsa Logs into Panels." In this patented technique, logs as small as four inches in diameter are radially cut into sectors having the same apex angle, each sector then being longitudinally sliced at its apex and arc to form a truncated piece having a trapezoidal cross-section, only a relatively small percentage of the wood being wasted. The pieces are thereafter fitted together in a complementary manner and interlaminated to form an integrated stock block which is dividable into panels.
The technique disclosed in my prior patent makes it possible to commercially exploit a broad range of balsa log diameters, running from small diameter logs cut from trees which take only 9 to 10 months to grow, to large diameter logs cut from more mature trees that take at least 5 to 8 years to grow. In this way, better use can be made of the available acreage. And because the logs are cut radially, the resultant area of the exposed surfaces is greater than that obtained with conventionally cut logs, thereby markedly reducing kiln drying time and its attendant costs. But even more important is the fact that the yield is exceptionally high; for, as compared to a traditional conversion which requires 60 logs of 12-inch diameter and 16 feet length to produce 1,000 board feet of balsa product, the technique disclosed in my prior patent yields the same amount of product from merely 20 such logs.
In my above-identified copending application Ser. No. 894,047, there is disclosed a technique for converting into lumber products logs of a diameter smaller than the smallest diameter which can be converted on a commercial scale by the technique disclosed in my prior patent; namely, logs cut from trees whose diameters lie in a range of about 11/2 inches to 4 inches.
It must be borne in mind that balsa grows at a fairly rapid rate, and while it takes at least 8 years for a tree to mature, the diameter of the tree after only 3 months is about 2 inches, and after 9 months about 4 inches. Young balsa trees are generally thickly planted, but most of these die off after the first two years; for the laws of natural selection doom all but the fittest or best-placed trees which survive and grow to maturity.
With a technique of the type disclosed in my copending application, one can harvest and convert balsa trees after a few months, well before natural selection takes over, whereby the yield from a given acreage is enormously increased. Since a single acre can easily support thousands of young trees in the 2 to 4 inch diameter range, a technique which utilizes without waste the whole log derived from such trees yields much more usable wood product than was hitherto possible. Because it takes no more than about three months for a balsa tree to reach a diameter of 2 inches with a usable log length of 8 feet, about four thousand trees can be harvested per acre three times a year, each tree giving 2 board feet of usable cylindrical wood substance. Thus one acre will provide 24,000 board feet per year, and in eight years the same acre will yield 192,000 board feet, a quantity far greater than the yield derivable from the traditional technique.
In the high yield technique disclosed in my copending application for converting balsa logs in a range of diameters from about 11/2 inches to 4 inches into rectangular panels and other lumber products, the raw logs are peeled to expose the wood, the peeled logs then being cut to a suitable length such as three feet to produce round pieces which are kiln-dried to a moisture content of about 12% or less. The dried pieces are then assembled into a block, the pieces being coated with a curable adhesive and being subjected to compression in orthogonal directions until the adhesive is cured and the pieces interlaminated to provide an integrated stock block. Finally, the stock block is divided into rectangular panels of the desired thickness.
In balsa wood panels of the type produced by the techniques disclosed in my prior patent and in my copending application, the panels are relatively rigid and may therefore be used in structural sandwich laminates created by bonding thin facings to the panels which then function as a core. Thus the Kohn et al. U.S. Pat. No. 3,325,037 discloses structural laminates of this type whose core is formed of end grain balsa wood, the laminates having an exceptionally high strength-to-weight ratio as well as excellent thermal insulation properties.
In contradistinction to such rigid panels, the concern of the present invention is with balsa wood panels that are inherently contourable; that is, with panels that can be conformed to curved surfaces for lamination thereto. No such panels have heretofore existed; for what has been known are contourable balsa wood blankets of the type disclosed in the Shook U.S. Pat. No. 3,540,967, composed of square individual balsa tiles attached to a fabric scrim which serves to interhinge the tiles whereby the blanket may be conformed to curved surfaces.
Such blankets, which are commercially available under the trademark "Contourkore," are useful in the construction of reinforced plastic boats and larger vessels, for they lend themselves to lamination between layers of reinforced fiberglass or other plastic material, thereby bringing about a distribution of weight favorable to high stability and buoyancy, as well as imparting stiffness to the structure.
In order to produce such contourable balsa wood blankets, one must start with a rigid panel of balsa wood and cut this panel into tiles which are then adhered to a common carrier or flexible scrim. This is a relatively complex and costly procedure.